Nestopia is an accuracy-focused emulator for the Nintendo Entertainment
System/Famicom (and Famicom Disk System). It is a computer program that
acts like a physical piece of hardware. In other words, this is a method
of preserving videogame history, with the welcome side effect of
allowing you to play NES games on a modern computer.

Nestopia uses highly optimized cycle exact emulation, allowing it to run
titles that rely on precise timing, many of which break under inferior
emulators.
